The present invention relates to the field of on-screen displays (OSD), particularly such as are employed to display information on a television (TV) picture screen.
In a TV receiver, it may be desirable to display directly on the picture screen information such as, for example, the selected channel number, a functional status message, a list of available programming options, or the time of day. Appropriate OSD signals corresponding to the information to be displayed may be generated by character or pattern generating OSD circuitry and applied to the luma/chroma processing portion of the TV receiver for display in the usual manner, possibly along with a picture produced in response to a received television signal.
Typically, the OSD signals are generated in synchronized relationship with timing pulses derived from the horizontal and vertical deflection signals, so as to locate the OSD in the desired portion of the picture screen. When a television signal is being received, such as off the air or from a tape or cable system, the horizontal and vertical deflection signals are synchronized with sync pulses derived from the received television signal by a sync separator. When the received television signal is present and sufficiently strong, the characters and symbols in the OSD will recur substantially in the same position on the picture screen from field to field and vertical character edges will not appear broken.
It may also be desirable to provide an OSD during certain times when no suitable television signal is being received by the TV receiver. This may be understood by considering that modern TV receivers may provide a number of viewer selectable options and features. Questions and prompting statements on the picture screen provide a convenient way of presenting the choices available and responding to selections made by a user. For example, a TV receiver may be arranged to cooperate with a number of accessory instruments, such as an antenna, a video cassette recorder (VCR), a cable distribution system, a computer and external loudspeakers. When the TV receiver is placed in a particular selection mode, an OSD may be used to present the choices for interconnecting various instruments in desired combinations. In another mode, the OSD may be used to request that a desired channel number be entered. The OSD may also provide a helpful explanation in ambiguous situations. For example, a TV receiver may be arranged to blank the picture screen when the receiver is tuned to a channel that is not providing a suitable television signal. The blank screen may be puzzling to a viewer unless, for example, the channel number and possibly an explanatory message are displayed by an OSD. Under the above and other similar conditions, either no suitable television signal is being received by the receiver or it is desirable that the display be independent of received signals.
However, at times when no suitable television signal is being received, there will be no proper separated sync signal available from the sync separator. Rather, the sync separator output will then provide a noisy signal. The deflection circuits in a typical TV receiver include an oscillator arrangement designed to be free-running in the absence of sync signals, usually at a slightly lower repetition rate than that produced when a proper sync signal is produced by the sync separator. The actual rate will be randomly affected by the noisy signal from the sync separator. Under such unsteady scanning conditions, the position of OSD characters and symbols will be likely to vary from one scanning line to another and from one scanning field to another, so that "jagged" edges and vertical "bounce" will be produced. This tends to reduce legibility and generally results in a displeasing and undesirable on-screen display.
It is possible to solve this problem by incorporating a substitute sync signal generator in the receiver to provide substitute sync signals to the deflection circuits when no suitable external signal is being received. However, this can considerably increase the cost of a receiver, particularly so, because the substitute sync signal generator tends to be rather complex in order to provide sync signals having the required stability.